Biomimetic co-assembly of gibbsite and mesophases: a novel synthesis route Tby devitrification of gel derived from hydrothermal aluminosilicate\CTAB Tsolution

The effect of an amphiphilic surfactanton the co-assembly of gibbsite and lowdimensional-order, liquid-crystalline mesophases using a hydrothermal-gelationdevitrification route was examined. Crystal growth by this method occurred either in three dimensions or was limited to only two dimensions. Gibbsite and mesophases were synthesized using a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), as template and a commercial mineral sample as inorganic precursor. The commercial mineral sample contained pyrophyllite, quartz, and minor kaolinite. The syntheses were made at pH 10 under hydrothermal conditions followed by equilibration at room temperature. The hydrothermally soluble portion settled at room temperature to form a translucent hydrous gel. This translucent gel turned white after drying on a glass substrate because of the following events based on chemical analysis, X-ray diffraction, and optical\electron microscopy: (1) gibbsite preferentially nucleated at the gel\air and gel\glass interfaces to form spherulites of tabular gibbsite crystals with entrapped droplets; (2) a ~26 basal-spacing, aluminate-encased, lamellar mesophase formed by 2D growth near the edge of the drying gel; and (3) residual solution in entrapped droplets within the gibbsite phase later devitrified abruptly into an optically isotropic material (an aluminosilicate gel possibly with minor mesophases) with a dendritic morphology. Formation of gibbsite and the lamellar mesophase was initially interface controlled, but later became 2D diffusion controlled as CTAB concentrations and micelle lengths were increased with crystallization time. A relatively high surfactant\water ratio of the drying gel might account for predominant crystallization of an aluminate-encased lamellar mesophase rather than the hexagonal mesophase known as the Mobil Composition Material MCM-41.